2009-06-21

Supressing RFI or Why won't my clock calibrate?

One of the primary things to watch out for in designing electronic circuits is noise suppression. Our microprocessors thrive on square waves, which can generate overwhelming broadband interference.

Any radio DXer can fill you in to great depth about RFI and its impact on their hobby. From the neighbor's electric fence to keep his horses in check to the lowly capacitive switched touch lamp or light dimmer, badly designed and/or failing circuits are his bane. As an example, here's what you need to do to the average wall wart to render it into a smooth, noise free power source for an active antenna.

In our WWVB clock, we are dealing with several noise sources that will likewise degrade the operation of our 60 kHz WWVB radio receiver. The Arduino has a 16 MHz clocked microprocessor, the LCD display serial interface has its own PIC microprocessor. Under these conditions, good circuit design includes establishment of a ground plane, and capacitors to bypass any unwanted RF noise to ground at the source. For our purposes, we need a 0.1uF capacitor on the microprocessor end of our CMMR-6P WWVB receiver's power supply cable, and a 0.1uF capacitor on the power supply to the LCD display.

The WWVB receiver board has already been supplied with power filtration on the other end of the cable.